Cast members of aluminum alloys advantageous in weight reduction, easy working to complicated shapes, production cost reduction, etc. are widely used for various parts. Particularly energy reduction and the improvement of fuel efficiency are required for automobiles, etc., and cast members of aluminum alloys constituting them are desired to have further reduced weight and higher quality. To have mechanical properties generally required for parts constituting vehicles, etc., casting aluminum alloys are required to have yield strength of about 200 MPa or more and elongation of about 3% or more, and particularly parts constituting automobile bodies, etc., which should have strength enough resistant to plastic deformation even when made thinner, are required to have yield strength of about 220 MPa or more.
It is known that the yield strength of metal materials such as aluminum alloys, etc. increases as their crystal grains become smaller. One of factors affecting the crystal grain sizes is a solidification rate; a higher solidification rate provides smaller crystal grains and a higher yield strength. To increase the solidification rate for a higher yield strength, it may be considered to make cast members thinner, and use a high-pressure die-casting method whose rate is higher than those of a low-pressure die-casting method and a gravity die-casting method. However, because the resultant castings have non-uniform shapes and sizes as well as casting defects, etc., the improvement of a yield strength only by increasing the solidification rate is limited.
Casting aluminum alloys include hypoeutectic Al—Si aluminum alloys such as JIS ADC12, AC4B, etc. However, the ADC12 alloy has as low yield strength as about 150 MPa in an as-cast state despite excellent castability, and the AC4B alloy needs a heat treatment after casting to have a yield strength of about 200 MPa. However, heat treatment increases a production cost because it increases the number of steps and energy consumption, and likely provides thin, complicated or large castings with deformation and strain, further increasing the cost to remove them.
Hyper-eutectic Al—Si alloys such as JIS ADC14 having a high yield strength without heat treatment are also known. Though this alloy has a yield strength of about 250 MPa in an as-cast state, ductility-decreasing, hard, brittle Si particles are easily crystallized because of the high Si content, so that it has extremely low elongation of less than about 1%, resulting in the limited applications of its cast members. Because the elongation of less than about 1% provides insufficient ductility, cast members are easily cracked and broken by impact when dropped.
Recently becoming used as casting aluminum alloys different from the Al—Si aluminum alloys to meet the requirement of higher quality are Al—Mg aluminum alloys such as JIS ADC5, ADC6, AC7A, etc. Though these aluminum alloys have excellent ductility without heat treatment, they have insufficient strength. For example, the ADC5 alloy has as low yield strength as about 190 MPa. In addition, the Al—Mg aluminum alloys are poorer than the Al—Si aluminum alloys in castability such as poorer melt flow, more misrun, larger solidification shrinkage, more shrinkage cavities, more cracks (hot cracks) on the surface, etc. In other words, the Al—Mg aluminum alloys do not have yield strength making up for cost increase for increasing castability.
As an attempt to improve the castability of Al—Mg aluminum alloys, JP 5-163546 A proposes a high-pressure die-casting aluminum alloy comprising 3.5-8.5% by weight of Mg, 1.5-4.0% by weight of Si, 0.3-1.0% by weight of Fe and 0.2-0.6% by weight of Mn, the balance being Al and inevitable impurities. Mg and Si synergistically contribute to increasing the strength and castability, preventing hot cracking. JP 5-163546 A describes that this aluminum alloy may contain Cr, Cu, Ti, Zr and Zn as impurities.
However, JP 5-163546 A fails to refer to yield strength and elongation, though it describes the hot cracking ratio, thermal expansion coefficient and tensile strength of the alloy. Presumption referring to the tensile strength, a typical mechanical property, reveals that the Al—Mg aluminum alloy of JP 5-163546 A has insufficient yield strength of about 180 MPa. Thus, conventional, Al—Si-based, or Al—Mg-based, casting aluminum alloys do not have sufficient elongation and yield strength in an as-cast state.